1. Field of the Invention
The present invention generally relates to the detection and location of obscured electrical conductors, and more particularly to an apparatus which adapts a conductor locator to be able to additionally detect passive resonant electronic markers.
2. Description of the Prior Art
Buried conduits are employed for supplying a wide variety of utilities, including pipelines for gas, water and sewage, and cables for telephone, power and television. It often becomes necessary to locate defective or damaged cables, pipes, etc., in order to repair or replace them. Conversely, it is important to know the approximate vicinity of such items in order to avoid disturbing them when digging or excavating for other purposes.
There are two primary techniques for location of such obscured conduits. The first requires that the conduit have (or be) a continuous electrical conductor along its length, such as telephone, power and television cables which not only have the copper conductors used to transmit the signal or power, but also typically have a ground shield surrounding the cable. The second technique, which may be used on pipes (such as gas and water mains) which do not have such a continuous conductor, requires the previous placement of an electronic marker(s) adjacent the conduit during its burial.
In the first of these techniques, a test signal (alternating current) is applied, directly or inductively, to the conductor, which then acts as an antenna and radiates the test signal along the length of the conduit. A locating apparatus is then used to detect the presence of the test signal, and the locator may further process the signal to determine the lateral direction to the conductor, and its depth. The earliest cable locators use a single sensor which detects a single null or peak (depending upon the orientation of the sensor) as the unit passes near the cable. Many later devices use two or more sensors that combine the signals to provide an indication of conductor proximity. The most common sensors are ferrite-core antennas, i.e., inductors. Several prior art locators are described in the following patents:
______________________________________ U.S. Pat. Nos. ______________________________________ 3,617,865 4,438,389 3,860,866 4,542,344 3,889,179 4,520,317 3,988,663 4,639,674 4,091,322 4,665,369 4,134,061 4,672,321 4,220,913 4,686,454 4,295,095 4,843,324 4,387,340 5,001,430 4,390,836 5,093,622 4,427,942 ______________________________________
In the second technique, the electronic markers may be active (e.g., have a battery to supply the signal), but passive markers are more common, having a capacitor and wire coil forming a resonant LC circuit. A given marker has only a single frequency (bandwidth centerline) which is hard-wired, and whose value depends upon the capacitance and inductance of the circuit. A transceiver having a radiating antenna and a pick-up antenna is used to detect passive markers. The radiating antenna intermittently outputs a signal having a frequency tuned to energize the marker. If there is a marker of the appropriate frequency within the vicinity of the transceiver, it absorbs a portion of the signal and re-radiates it. During the periods between signal output by the transceiver, the pick-up antenna listens for any re-radiated signal, and notifies the user if one is found, and usually provides an indication of signal strength. Several prior art markers are described in the following patents:
______________________________________ U.S. Pat. Nos. ______________________________________ 3,836,842 4,873,533 4,334,227 4,947,012 4,712,094 5,017,415 4,761,656 5,045,368 ______________________________________
Generally speaking, the methods used by the locators' circuitry differ depending on whether a conductor or a marker is being searched for. Nevertheless, there are hybrid systems wherein a signal is applied to a buried conductor, and coupled through the conductor to one or more markers buried adjacent the conductor. See, e.g., U.S. Pat. No. 4,119,908, 4,767,237 and 4,862,088. Also, in U.S. Pat. No. 4,866,388, a marker is used to couple one conductor to another, so that the test signal may be conveyed to the second conductor without a direct connection.
There are several difficulties, however, in attempting to combine conductor and marker locators. First of all, most passive electronic markers are environmentally sealed (electrically insulated) and so cannot be activated by application of an alternating current, i.e., they are not connected to a conductor that runs the length of the conduit. Indeed, markers are often specifically used to avoid the presence of a continuous conductor along the conduit, such as in natural gas distribution where a lightning strike could progress down such a conductor and ignite a leak. The passive, isolated nature of the markers presents a problem since most conductor locators do not themselves transmit any signal which could be re-radiated by the marker. Of course, the marker could be modified to allow direct physical connection to the conductor, as described in U.S. Pat. No. 4,862,088. In that system, a marker locator may be placed at any point (above ground) along a cable having a metallic screen which is coupled to a resonant marker. This system is undesirable, however, since it requires significant modification to the marker design, adding cost, and it requires more time and effort in installation; furthermore, it cannot be used to trace conductors which are not coupled to resonant circuits. It would also be very difficult to use this device with certain improved marker designs, such as the ball marker shown in U.S. Pat. No. 4,712,094, since that marker is designed for movement relative to the conduit (which would break the electrical connection in the '088 device), and the marker body is filled with water.
The primary limitation, however, in such a combination of marker and conductor locators relates to the frequency conventions used to identify different markers associated with different types of utilities. Five distinct frequencies have been designated: 83.0 kHz for gas; 101.4 kHz for telephone; 121.6 kHz for sewage; 145.7 kHz for water; and 169.8 kHz for power (the markers are also usually color-coded). In this manner, a service technician searching for, say, a gas line, cannot accidentally activate a telephone marker since his transmitter will only be sending out an 83 kHz signal, which is outside the bandwidth of a telephone marker tuned for 101.4 kHz. Of course, these frequencies have been designated by convention, and they should not be construed in a limiting sense with respect to the present invention.
The problem, then, is that the same signal induced on the conductor cannot be used to activate the marker unless it is at the appropriate frequency. This is compounded by the fact that most conductor locators use two frequencies (577 Hz or 33 kHz) other than those used with markers. Therefore, conventional conductor locators (even if they were adapted to send a downward signal) would still not detect most markers since the markers would be tuned to the wrong frequency. Markers may be provided which respond to more than one frequency, but they cost more and may lead to false locates when searching for a different type of utility from that found. In U.S. Pat. Nos. 4,119,908, 4,866,388 and 4,767,237, a single locator is used for both conductor and marker location, but the locator can only detect conductors which radiate the signal tuned to the specific frequency of the marker, i.e., the locator requires a (separate) transmitter which is tunable to each frequency in the family of markers which are desired to be locatable. This adds great expense, particularly for users who only want conductor, and not marker, locating ability. It would, therefore, be desirable to devise an apparatus which would allow for the detection of both markers and conductors, and yet not require modification of the basic conductor locator or the marker design. It would further be advantageous if the device could detect both structures simultaneously.